Systems and methods for performing neurophysiologic monitoring

ABSTRACT

The present invention relates to a system and methods generally aimed at surgery. More particularly, the present invention is directed at a system and related methods for performing surgical procedures and assessments involving the use of neurophysiology.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.14/856,525, filed Sep. 16, 2015, which claims the benefit of priorityfrom commonly owned and U.S. Provisional Patent Application No.62/051,141, entitled “Systems and Methods for PerformingNeurophysiologic Monitoring During Spine Surgery,” filed on Sep. 16,2014, and U.S. Provisional Patent Application No. 62/136,760, entitled“Method for improved detection of low level SSEP signals,” filed on Mar.23, 2015, the entire contents of both which is hereby expresslyincorporated by reference into this disclosure as if set forth in itsentirety herein.

FIELD

The present invention relates to a system and methods generally aimed atsurgery. More particularly, the present invention is directed at asystem and related methods for performing neurophysiologic assessmentsduring surgical procedures.

BACKGROUND

The spinal column is a highly complex system of bones and connectivetissues that provide support for the body and protect the delicatespinal cord and nerves. The spinal column includes a series of vertebralbodies stacked one atop the other, each vertebral body including aninner or central portion of relatively weak cancellous bone and an outerportion of relatively strong cortical bone. Situated between eachvertebral body is an intervertebral disc that cushions and dampenscompressive forces exerted upon the spinal column. A vertebral canalcontaining the spinal cord is located behind the vertebral bodies.

There are many types of spinal column disorders including scoliosis(abnormal lateral curvature of the spine), excess kyphosis (abnormalforward curvature of the spine), excess lordosis (abnormal backwardcurvature of the spine), spondylolisthesis (forward displacement of onevertebra over another), and other disorders caused by abnormalities,disease or trauma, such as ruptured or slipped discs, degenerative discdisease, fractured vertebrae, and the like. Patients that suffer fromsuch conditions usually experience extreme and debilitating pain as wellas diminished nerve function.

Neurophysiologic monitoring has become an increasingly important adjunctto surgical procedures where neural tissue may be at risk. Spinalsurgery, in particular, involves working close to delicate tissue in andsurrounding the spine, which can be damaged in any number of differentways. When spinal cord monitoring is required, somatosensory evokedpotential (SSEP) monitoring is often chosen. SSEP monitoringtraditionally involves complex analysis and specially trainedneurophysiologists are generally called upon to perform the monitoring.Even though performed by specialists, interpreting the complex waveformsin this fashion is nonetheless disadvantageously prone to human errorand can be disadvantageously time consuming, adding to the duration ofthe operation and translating into increased healthcare costs. Even morecostly is the fact that the neurophysiologist is required in addition tothe actual surgeon performing the spinal operation. Past developmentshave attempted to solve these challenges in various ways. One suchdevelopment is so-called automated or surgeon-driven SSEPs monitoring.

For some time, surgeon-driven and traditional neuromonitoring systemshave co-existed and have experienced some of the same challenges inperforming intraoperative neuromonitoring. One such challenge is thatneurophysiologic signals are typically sub-microvolt evoked potentialsthat are hard to resolve in an “electrically hostile” environment suchas an operating room. What is needed are systems and methods forimproved SSEP data acquisition that provides meaningful data to user.According to a broad aspect of the present invention, there are providedmethods and techniques to enhance, facilitate, and/or simplify theprocess of detecting neurophysiologic signals (e.g. SSEP signals)particularly in the presence of noise including ambient electricalactivity and non-evoked biopotentials.

SUMMARY OF THE INVENTION

The present invention includes systems and methods to evaluate thehealth and status of the lower motor neural pathway before, during andafter the establishment of an operative corridor through (or near) anyof a variety of tissues having such neural structures which, ifcontacted or impinged, may otherwise result in neural impairment for thepatient. It is expressly noted that, although described herein largelyin terms of use in lateral lumbar spinal surgery, the system and methodsof the present disclosure are suitable for use in any number ofadditional spinal surgeries including posterior, posterolateral,anterior, anterolateral lumbar spinal surgeries as well as thoracic andthoracolumbar spinal surgeries. Indeed, the invention of the presentdisclosure is suitable for use in any number of additional surgicalprocedures wherein tissue having significant neural structures must bepassed through (or near) in order to establish an operative corridor.

According to another broad aspect, the present invention includes acontrol unit, a patient module, and a plurality of surgical accessoriesadapted to couple to the patient module. The control unit includes apower supply and is programmed to receive user commands, activatestimulation in a plurality of predetermined modes, process signal dataaccording to defined algorithms, display received parameters andprocessed data, and monitor system status. The patient module is incommunication with the control unit. The patient module is within thesterile field. The patient module includes signal conditioningcircuitry, stimulator drive circuitry, and signal conditioning circuitryrequired to perform said stimulation in said predetermined modes. Thepatient module includes a processor programmed to perform a plurality ofpredetermined functions including at least two of neuromuscular pathwayassessment, non-evoked monitoring, static pedicle integrity testing,dynamic pedicle integrity testing, nerve proximity detection, manualmotor evoked potential monitoring, automatic motor evoked potentialmonitoring, transcutaneous nerve root testing, manual somatosensoryevoked potential monitoring, automatic somatosensory evoked potentialmonitoring, and surgical correction planning and assessment.

According to still another broad aspect, the present invention includesa processing unit programmed to perform a plurality of predeterminedfunctions using said instrument including at least two of neuromuscularpathway assessment, static pedicle integrity testing, dynamic pedicleintegrity testing, nerve proximity detection, transcutaneous nerve roottesting, non-evoked monitoring, motor evoked potential monitoring,somatosensory evoked potential monitoring, and surgical correctionplanning and assessment. The processing system has a pre-establishedprofile for at least one of said predetermined functions so as tofacilitate the initiation of said at least one predetermined function.

According to one aspect of the present disclosure, there is provided ahunting algorithm executable on the control unit of the neurophysiologicmonitoring system (for example, the neurophysiologic monitoring systemshown and described below) that finds a minimum rejection threshold thatwill quickly find a significant response with a minimum number ofresponses to provide a significant, well-resolved SSEP waveform for thesurgeon without the need for highly trained personnel to be present toacquire or interpret the waveforms. It is to be appreciated while thehunting algorithm described below is described with respect tosomatosensory evoked potentials, it is equally applicable to allneurophysiologic modalities, including but not limited to motor evokedpotentials (MEP) and transcutaneous, trans-abdominal evoked potentials.It will also be appreciated that the hunting algorithm described belowis applicable to not just spine procedures but any procedure in whichone or more aspects of the nervous system are at risk of permanent ortransient injury or damage.

BRIEF DESCRIPTION OF THE DRAWINGS

Many advantages of the present invention will be apparent to thoseskilled in the art with a reading of this specification in conjunctionwith the attached drawings, wherein like reference numerals are appliedto like elements and wherein:

FIG. 1 is a block diagram of an example neurophysiologic monitoringsystem capable of conducting multiple nerve and spinal cord monitoringfunctions including but not necessarily limited to neuromuscularpathway, bone integrity, nerve detection, nerve pathology (evoked orfree-run EMG), lower motor pathway, MEP, and SSEP assessments;

FIG. 2 is a perspective view showing examples of several components ofthe system of FIG. 1;

FIG. 3 is a graph illustrating a plot of a single pulse stimulationcurrent signal capable of producing a neuromuscular response (EMG) ofthe type shown in FIG. 5;

FIG. 4 is a graph illustrating [a] plot of a stimulation current signalcomprising a train of pulses capable of producing a neuromuscularresponse (EMG) of the type shown in FIG. 5;

FIG. 5 is a graph illustrating a plot of the neuromuscular response of agiven myotome over time based on a stimulation signal (such as shown ineither Fit. 4 or FIG. 5);

FIG. 6 is a perspective view of an example of a control unit formingpart of the system of FIG. 1;

FIGS. 7-9 are perspective, top, and side views, respectively, of anexample of a patient module forming part of the system of FIG. 1;

FIG. 10 is a top view of an electrode harness forming part of the systemof FIG. 1;

FIGS. 11A-11C are side views of various examples of harness portsforming part of the system of FIG. 1;

FIG. 12 is a plan view of an example of a label affixed to an electrodeconnector forming part of the system of FIG. 1;

FIGS. 13A-13B are top views of examples of electrode caps forming partof the system of FIG. 1;

FIGS. 14A-14B are screenshots of an example embodiment of an electrodetest screen forming part of the system of FIG. 1;

FIG. 15 is a perspective view of one embodiment of a stimulator formingpart of the system of FIG. 1;

FIG. 16 is a perspective view of a second embodiment of a stimulatorforming part of the system of FIG. 1;

FIG. 17 is a perspective view of the stimulators of FIGS. 15 and 16coupled together for use;

FIGS. 18-19 are perspective views of an example of a secondary displayforming part of the system of FIG. 1;

FIG. 20 is a screenshot of an example embodiment of a profile screenforming part of the system of FIG. 1;

FIG. 21 is a screenshot of an example of a Twitch Test monitoring screenforming part of the system of FIG. 1;

FIG. 22 is a screenshot of an example embodiment of a Basic StimulationEMG monitoring screen forming part of the system of FIG. 1;

FIG. 23 is a screenshot of an example embodiment of a DynamicStimulation EMG monitoring screen forming part of the system of FIG. 1;

FIG. 24 is a screenshot of an example embodiment of a Nerve SurveillanceEMG monitoring screen forming part of the system of FIG. 1;

FIG. 25 is a screenshot of an example embodiment of a Manual MEPmonitoring screen forming part of the system of FIG. 1;

FIG. 26 is a screenshot of an example embodiment of an Automatic MEPmonitoring screen forming part of the system of FIG. 1;

FIG. 27 is a screenshot of an example embodiment of a TCNR Alertmonitoring screen forming part of the system of FIG. 1;

FIG. 28 is a screenshot of an example embodiment of a TCNR Thresholdmonitoring screen forming part of the system of FIG. 1;

FIG. 29 is a screenshot of an example embodiment of a first surgicalcorrection planning and assessment screen forming part of the system ofFIG. 1;

FIG. 30 is a screenshot of an example embodiment of second surgicalcorrection planning and assessment screen forming part of the system ofFIG. 1;

FIG. 31 is a screenshot of an example embodiment of an SSEP profileselection screen forming part of the system of FIG. 1;

FIG. 32 is a screenshot of an example embodiment of an SSEP ManualStimulus Mode setting with a Left Ulnar Nerve (LUN) breakout screenforming part of the system of FIG. 1;

FIG. 33 is a screenshot of one embodiment of an SSEP Manual Run screenforming part of system of FIG. 1;

FIG. 34 is a screenshot of a second embodiment of an SSEP Manual Runscreen forming part of the system of FIG. 1;

FIG. 35 is a screenshot of a third embodiment of an SSEP Manual Runscreen forming part of the system of FIG. 1;

FIG. 36 is a screenshot of a fourth embodiment of an SSEP Manual Runscreen forming part of the system of FIG. 1;

FIG. 37 is a screenshot of one embodiment of an SSEP Automatic Testscreen forming part of the system of FIG. 1;

FIG. 38 is a screenshot of one embodiment of an SSEP Automatic Runscreen forming part of the system of FIG. 1;

FIG. 39 is a screenshot of a second embodiment of an SSEP Automatic Runscreen forming part of the system of FIG. 1;

FIG. 40 is a screenshot of a third embodiment of an SSEP Automatic Runscreen forming part of the system of FIG. 1;

FIG. 41 is a screenshot of a fourth embodiment of an SSEP Automatic Runscreen forming part of the system of FIG. 1;

FIG. 42 is a flowchart illustrating a method by which a baseline huntingalgorithm determines optimized SSEP acquisition conditions; and

FIG. 43 is a flowchart illustrating a method by which a data set sizingalgorithm determines the number of SSEP stimulations required to achievea well-resolved SSEP waveform.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Illustrative embodiments of the invention are described below. In theinterest of clarity, not all features of an actual implementation aredescribed in this specification. It will of course be appreciated thatin the development of any such actual embodiment, numerousimplementation-specific decisions must be made to achieve thedevelopers' specific goals, such as compliance with system-related andbusiness-related constraints, which will vary from one implementation toanother. Moreover, it will be appreciated that such a development effortmight be complex and time-consuming, but would nevertheless be a routineundertaking for those of ordinary skill in the art having the benefit ofthis disclosure. The systems disclosed herein boast a variety ofinventive features and components that warrant patent protection, bothindividually and in combination. The systems and methods describedherein boast a variety of inventive features and components that warrantpatent protection, both individually and in combination.

A neurophysiologic monitoring system 10 is described herein and iscapable of performing a number of neurophysiological and/or guidanceassessments at the direction of the surgeon (and/or other members of thesurgical team). By way of example only, FIGS. 1-2 illustrate the basiccomponents of the system 10. The system comprises a control unit 12(including a main display 34 preferably equipped with a graphical userinterface (GUI) and a processing unit 36 that collectively contain theessential processing capabilities for controlling the system 10), apatient module 14, a stimulation accessory (e.g. a stimulation probe 16,stimulation clip 18 for connection to various surgical instruments, aninline stimulation hub 20, and stimulation electrodes 22), and aplurality of recording electrodes 24 for detecting electricalpotentials.

The stimulation accessories may be in the form of various probe devicesthat are themselves inserted into the stimulation site, clips thatattach and deliver stimulation signals to standard instruments that areused at various times throughout a procedure and surface electrodes. Thestimulation clip 18 may be used to connect any of a variety of surgicalinstruments to the system 10, including, but not necessarily limited toa pedicle access needle 26, k-wire 27, tap 28, dilator(s) 30, tissueretractor 32, etc. One or more secondary feedback devices (e.g.secondary display 46 in FIGS. 20-21) may also be provided for additionalexpression of output to a user and/or receiving input from the user.

In one embodiment, the system 10 may be configured to execute any of thefunctional modes including, but not necessarily limited to,neuromuscular pathway assessment (“Twitch Test”), non-evoked monitoring(“Free-run EMG”), static pedicle integrity testing (“Basic StimulatedEMG”), dynamic pedicle integrity testing (“Dynamic Stimulated EMG”),nerve proximity detection (“XLIF®”), motor evoked potential monitoring(“MEP Manual” and “MEP Automatic”), transcutaneous nerve root testing(“TCNR Alert” and “TCNR Threshold”), somatosensory evoked potentialmonitoring (“SSEP Manual” and “SSEP Automatic”), and surgical correctionplanning and assessment. The system 10 may also be configured forperformance in any of the lumbar, thoracolumbar, and cervical regions ofthe spine.

The basis for performing many of these functional modes (e.g. TwitchTest, Basic Stimulated EMG, Dynamic Stimulated EMG, XILF, MEP Manual,MEP Automatic, TCNR Alert, and TCNR Threshold) is the assessment ofevoked responses of the various muscles myotomes monitored by the system10 in relation to a stimulation signal transmitted by the system 10 (viapatient module 14). The assessment of the evoked responses can be anysuitable means of sensing physical motion of a muscle, for example viamechanomyography (MMG) which in one embodiment entails using anaccelerometer or other similar device for detecting mechanical movementof a muscle or via electromyography (EMG) which is described in detailherein. This is illustrated in FIGS. 3-5, wherein FIG. 5 illustrates theresulting EMG waveform of a monitored myotome in response to one of theexample stimulation signals represented in FIG. 3 and FIG. 4. The EMGresponses provide a quantitative measure of the nerve depolarizationcaused by the electrical stimulus. One way to characterize the EMGresponse is by a peak-to-peak voltage of V_(pp)=V_(max)−V_(min), asshown in FIG. 5. Nerve tissues have characteristic threshold currentlevels (I_(thresh)) at which they will depolarize and result in adetectable muscle activity. Below this threshold current level, astimulation signal will not evoke a significant EMG response. Accordingto one embodiment, a significant EMG response may be defined as having aV_(pp) of approximately 100 uV. Thus, the lowest stimulation currentnecessary to evoke an EMG response of the threshold voltage(V_(thresh)), 100 uV in this example, may be called I_(thresh). Thegreater the degree of electrical communication between a stimulationsignal and a nerve, the lower I_(thresh) will be. Conversely, the lowerthe degree of electrical communication between a stimulation signal anda nerve, the greater I_(thresh) will be. Thus determining I_(thresh),and/or monitoring changes in I_(thresh) over time, may provide valuableinformation when nerve tissues are at risk during a surgical procedure,as will be discussed in more detail below. By way of example, anexcessively high I_(thresh) or an increase over a previous measurementduring MEP testing may indicate a problem in the spinal cord or otherportion of the motor pathway inhibiting transmission (communication) ofthe stimulation signal to the nerve. Meanwhile, during the BasicStimulated EMG or Dynamic Stimulated EMG modes and the XLIF mode, a lowI_(thresh) value may indicate a breach in the pedicle allowing theelectrical signal to transmit through the pedicle, or the closeproximity of a nerve to the stimulation source, respectively. Armed withthe useful information conveyed by I_(thresh), the surgeon may detect aproblem or potential problem early and then act to avoid and/or mitigatethe problem. The neurophysiology system 10 may quickly and accuratelydetermine I_(thresh) under the direction and operation of the surgeon(if desired) and convey the useful information I_(thresh) contains in asimple and easily comprehensible manner for interpretation by thesurgeon.

Before further addressing the various functional modes of the surgicalsystem 10, the hardware components and features of the system 10 will bedescribe in further detail. The control unit 12 of the system 10,illustrated by way of example only in FIG. 6, includes a main display 34and a processing unit 36, which collectively contain the essentialprocessing capabilities for controlling the system 10. The main display34 is preferably equipped with a graphical user interface (GUI) capableof graphically communicating information to the user and receivinginstructions from the user. The processing unit 36 contains computerhardware and software that commands the stimulation source (e.g. patientmodule 14, FIGS. 7-9), receives digital and/or analog signals and otherinformation from the patient module 14, processes EMG and SSEP responsesignals, and displays the processed data to the user via the display 34.The primary functions of the software within the control unit 12 includereceiving user commands via the touch screen main display 34, activatingstimulation in the appropriate mode (Twitch Test, Basic Stimulated EMG,Dynamic Stimulated EMG, XLIF, MEP Manual, MEP Automatic, TCNR Alert,TCNR Threshold, SSEP Manual, and SSEP Automatic), processing signal dataaccording to defined algorithms, displaying received parameters andprocessed data, and monitoring system status. According to one exampleembodiment, the main display 34 may comprise a 15″ LCD display equippedwith suitable touch screen technology and the processing unit 36 maycomprise a 2 GHz. The processing unit 36 shown in FIG. 6 furtherincludes a powered USB port 38 for connection to the patient module 14,a media drive 40 (e.g. CD, CD-RW, DVD, DVD-RW, etc. . . . ), a networkport, wireless network card, and a plurality of additional ports 42(e.g. USB, IEEE 1394, infrared, etc. . . . ) for attaching additionalaccessories, such as for example only, navigated guidance sensors,auxiliary stimulation anodes, and external devices (e.g. printer,keyboard, mouse, etc. . . . ). Preferably, during use the control unit12 sits near the surgical table but outside the surgical field, such asfor example, on a table top or a mobile stand. It will be appreciated,however, that if properly draped and protected, the control unit 12 maybe located within the surgical (sterile) field.

The patient module 14, shown by way of example only in FIGS. 4-6, iscommunicatively linked to the control unit 12. In this embodiment thepatient module 14 is communicatively linked with and receives power fromthe control unit 12 via a USB data cable 44. However, it will beappreciated that the patient module 14 may be supplied with its ownpower source and other known data cables, as well as wirelesstechnology, may be utilized to establish communication between thepatient module 14 and control unit 12. The patient module 14 contains adigital communications interface to communicate with the control unit12, as well as the electrical connections to all recording andstimulation electrodes, signal conditioning circuitry, stimulator driveand steering circuitry, and signal conditioning circuitry required toperform all of the functional modes of the system 10, including but notnecessarily limited to Twitch Test, Free-run EMG, Basic Stimulated EMG,Dynamic Stimulated EMG, XLIF, MEP Manual and MEP Automatic, TCNR Alert,TCNR Threshold, SSEP Manual, and SSEP Automatic. In one example, thepatient module 14 includes thirty-two recording channels and elevenstimulation channels. A display (e.g. an LCD screen) may be provided onthe face of the patient module 14, and may be utilized for showingsimple status readouts (for example, results of a power on test, theelectrode harnesses attached, and impedance data, etc. . . . ) or moreprocedure related data (for example, a stimulation threshold result,current stimulation level, selected function, etc. . . . ). The patientmodule 14 may be positioned near the patient in the sterile field duringsurgery. By way of example, the patient module 14 may be attached to bedrail with the aid of a hook 48 attached to, or forming a part of, thepatient module 14 casing.

With reference to FIGS. 7-9, patient module 14 comprises a multitude ofports and indicators for connecting and verifying connections betweenthe patient module 14 and other system components. A control unit port50 is provided for data and power communication with the control unit12, via USB data cable 44 as previously described. There are fouraccessory ports 52 provided for connecting up to the same number ofsurgical accessories, including, but not necessarily limited to,stimulation probe 16, stimulation clip 18, inline stimulation hub 20,and navigated guidance sensor (or tilt sensor) 54. The accessory ports52 include a stimulation cathode and transmit digital communicationsignals, tri-color LED drive signals, button status signals,identification signals, and power between the patient module 14 and theattached accessory. A pair of anode ports 56, preferably comprising 2wire DIN connectors, may be used to attach auxiliary stimulation anodesshould it become desirable or necessary to do so during a procedure. Apair of USB ports 58 are connected as a USB hub to the control unit 12and may be used to make any number of connections, such as for exampleonly, a portable storage drive.

As soon as a device is plugged into any one of ports 50, 52, 56, or 58,the system 10 automatically performs a circuit continuity check toensure the associated device will work properly. Each device forms aseparate closed circuit with the patient module such that the devicesmay be checked independent of each other. If one device is not workingproperly the device may be identified individually while the remainingdevices continue indicate their valid status. An indicator LED isprovided for each port to convey the results of the continuity check tothe user. Thus, according to the example embodiment of FIGS. 7-9, thepatient module 14 includes one control unit indicator 60, four accessoryindicators 62, two anode indicators 64, and two USB indicators 66.According to a preferred embodiment, if the system detects an incompletecircuit during the continuity check, the appropriate indicator will turnred alerting the user that the device might not work properly. On theother hand, if a complete circuit is detected, the indicator will appeargreen signifying that the device should work as desired. Additionalindicator LEDs are provided to indicate the status of the system and theMEP stimulation. The system indicator 68 will appear green when thesystem is ready and red when the system is not ready. The MEP stimindicator 70 lights up when the patient module is ready to deliver andMEP stimulation signal. In one embodiment, the MEP stim indicator 68appears yellow to indicate a ready status.

To connect the array of recording electrodes 24 and stimulationelectrodes 22 utilized by the system 10, the patient module 14 alsoincludes a plurality of electrode harness ports. In the embodimentshown, the patient module 14 includes an EMG/MEP harness port 72, SSEPharness port 74, an Auxiliary harness port 76 (for expansion and/orcustom harnesses; e.g. a TCNR harness). Each harness port 72, 74, and 76includes a shaped socket 78 that corresponds to a matching shapedconnector 82 on the appropriate electrode harness 80. In addition, thesystem 10 may preferably employ a color code system wherein eachmodality (e.g. EMG, EMG/MEP, and SSEP) has a unique color associatedwith it. By way of example only and as shown herein, EMG monitoring(including, screw tests, detection, and nerve retractor) may beassociated with the color green, MEP monitoring with the color blue, andSSEP monitoring may be associated with the color orange. Thus, eachharness port 72, 74, 76 is marked with the appropriate color which willalso correspond to the appropriate harness 80. Utilizing the combinationof the dedicated color code and the shaped socket/connector interfacesimplifies the setup of the system, reduces errors, and can greatlyminimize the amount of pre-operative preparation necessary. The patientmodule 14, and especially the configuration of quantity and layout ofthe various ports and indicators, has been described according to oneexample embodiment of the present invention. It should be appreciated,however, that the patient module 14 could be configured with any numberof different arrangements without departing from the scope of theinvention.

As mentioned above, to simplify setup of the system 10, all of therecording electrodes 24 and stimulation electrodes 22 that are requiredto perform one of the various functional modes (including a commonelectrode 23 providing a ground reference to pre-amplifiers in thepatient module 14, and an anode electrode 25 providing a return path forthe stimulation current) are bundled together and provided in singleelectrode harness 80, as illustrated, by way of example only, in FIG.10. Depending on the desired function or functions to be used during aparticular procedure, different groupings of recoding electrodes 24 andstimulation electrodes 22 may be required. By way of example, the SSEPfunction requires more stimulating electrodes 22 than either the EMG orMEP functions, but also requires fewer recording electrodes than eitherof the EMG and MEP functions. To account for the differing electrodeneeds of the various functional modes, the system 10 may employdifferent harnesses 80 tailored for the desired modes. According to oneembodiment, three different electrode harnesses 80 may be provided foruse with the system 10, an EMG harness, an EMG/MEP harness, and an SSEPharness.

At one end of the harness 80 is the shaped connector 82. As describedabove, the shaped connector 82 interfaces with the shaped socket 72, 74,or 76 (depending on the functions harness 80 is provided for). Eachharness 80 utilizes a shaped connector 82 that corresponds to theappropriate shaped socket 72, 74, 76 on the patient module 14. If theshapes of the socket and connector do not match the harness 80,connection to the patient module 14 cannot be established. According toone embodiment, the EMG and the EMG/MEP harnesses both plug into theEMG/MEP harness port 72 and thus they both utilize the same shapedconnector 82. By way of example only, FIGS. 11A-11C illustrate thevarious shape profiles used by the different harness ports 72, 74, 76and connectors 82. FIG. 11A illustrates the half circular shapeassociated with the EMG and EMG/MEP harness and port 72. FIG. 11Billustrates the rectangular shape utilized by the SSEP harness and port74. Finally, FIG. 11C illustrates the triangular shape utilized by theAuxiliary harness and port 76. Each harness connector 82 includes adigital identification signal that identifies the type of harness 80 tothe patient module 14. At the opposite end of the electrode harness 80are a plurality of electrode connectors 102 linked to the harnessconnector 82 via a wire lead. Using the electrode connector 102, any ofa variety of known electrodes may be used, such as by way of exampleonly, surface dry gel electrodes, surface wet gel electrodes, and needleelectrodes.

To facilitate easy placement of scalp electrodes used during MEP andSSEP modes, an electrode cap 81, depicted by way of example only in FIG.13A may be used. The electrode cap 81 includes two recording electrodes23 for SSEP monitoring, two stimulation electrodes 22 for MEPstimulation delivery, and an anode 23. Graphic indicators may be used onthe electrode cap 81 to delineate the different electrodes. By way ofexample, lightning bolts may be used to indicate a stimulationelectrode, a circle within a circle may be used to indicate recordingelectrodes, and a stepped arrow may be used to indicate the anodeelectrode. The anode electrode wire is colored white to furtherdistinguish it from the other electrodes and is significantly longerthat the other electrode wires to allow placement of the anode electrodeon the patient's shoulder. The shape of the electrode cap 81 may also bedesigned to simplify placement. By way of example only, the cap 81 has apointed end that may point directly toward the patient's nose when thecap 81 is centered on the head in the right orientation. A single wiremay connect the electrode cap 81 to the patient module 14 or electrodeharness 80, thereby decreasing the wire population around the upperregions of the patient. Alternatively, the cap 81 may be equipped with apower supply and a wireless antenna for communicating with the system10. FIG. 13B illustrates another example embodiment of an electrode cap83 similar to cap 81. Rather than using graphic indicators todifferentiate the electrodes, colored wires may be employed. By way ofexample, the stimulation electrodes 22 are colored yellow, the recordingelectrodes 24 are gray, and the anode electrode 23 is white. The anodeelectrode is seen here configured for placement on the patient'sforehead. According to an alternate embodiment, the electrode cap (notshown) may comprise a strap or set of straps configured to be worn onthe head of the patient. The appropriate scalp recording and stimulationsites may be indicated on the straps. By way of example, the electrodecap may be imbued with holes overlying each of the scalp recording sites(for SSEP) and scalp stimulation sites (for MEP). According to a furtherexample embodiment, the border around each hole may be color coded tomatch the color of an electrode lead wire designated for that site. Inthis instance, the recording and stimulation electrodes designated forthe scalp are preferably one of a needle electrode and a corkscrewelectrode that can be placed in the scalp through the holes in the cap.

In addition to or instead of color coding the electrode lead wires todesignated intended placement, the end of each wire lead next to theelectrode connector 102 may be tagged with a label 86 that shows ordescribes the proper positioning of the electrode on the patient. Thelabel 86 preferably demonstrates proper electrode placement graphicallyand textually. As shown in FIG. 12, the label may include a graphicimage showing the relevant body portion 88 and the precise electrodeposition 90. Textually, the label 86 may indicate the side 100 andmuscle (or anatomic location) 96 for placement, the function of theelectrode (e.g. stimulation, recording channel, anode, and reference—notshown), the patient surface (e.g. anterior or posterior), the spinalregion 94, and the type of monitoring 92 (e.g. EMG, MEP, SSEP, by way ofexample, only). According to one embodiment (set forth by way of exampleonly), the electrode harnesses 80 are designed such that the variouselectrodes may be positioned about the patient (and preferably labeledaccordingly) as described in Table 1 for Lumbar EMG, Table 2 forCervical EMG, Table 3 for Lumbar/Thoracolumbar EMG and MEP, Table 4 forCervical EMG and MEP, Table 5 for TCNR, and Table 6 for SSEP:

TABLE 1 Lumbar EMG Electrode Type Electrode Placement Spinal LevelGround Upper Outer Thigh — Anode Latissimus Dorsi — Stimulation Knee —Recording Left Tibialis Anterior L4, L5 Recording Left Gastroc. MedialisS1, S2 Recording Left Vastus Medialis L2, L3, L4 Recording Left BicepsFemoris L5, S1, S2 Recording Right Biceps Femoris L5, S1, S2 RecordingRight Vastus Medialis L2, L3, L4 Recording Right Gastroc. Medialis S1,S2 Recording Right Tibialis Anterior L4, L5

TABLE 2 Cervical EMG Electrode Type Electrode Placement Spinal LevelGround Shoulder — Anode Mastoid — Stimulation Inside Elbow — RecordingLeft Triceps C7, C8 Recording Left Flexor Carpi Radialis C6, C7, C8Recording Left Deltoid C5, C6 Recording Left Trapezius C3, C4 RecordingLeft Vocal Cord RLN Recording Right Vocal Cord RLN Recording RightTrapezius C3, C4 Recording Right Deltoid C5, C6 Recording Right FlexorCarpi Radialis C6, C7, C8 Recording Right Triceps C7, C8

TABLE 3 Lumbar/Thoracolumbar EMG + MEP Electrode Type ElectrodePlacement Spinal Level Ground Upper Outer Thigh — Anode Latissimus Dorsi— Stimulation Knee — Recording Left Tibialis Anterior L4, L5 RecordingLeft Gastroc. Medialis S1, S2 Recording Left Vastus Medialis L2, L3, L4Recording Left Biceps Femoris L5, S1, S2 Recording Left APB-ADM C6, C7,C8, T1 Recording Right APB-ADM C6, C7, C8, T1 Recording Right BicepsFemoris L5, S1, S2 Recording Right Vastus Medialis L2, L3, L4 RecordingRight Gastroc. Medialis S1, S2 Recording Right Tibialis Anterior L4, L5

TABLE 4 Cervical EMG + MEP Electrode Type Electrode Placement SpinalLevel Ground Shoulder — Anode Mastoid — Stimulation Inside Elbow —Recording Left Tibialis Anterior L4, L5 Recording Left Flexor CarpiRadialis C6, C7, C8 Recording Left Deltoid C5, C6 Recording LeftTrapezius C3, C4 Recording Left APB-ADM C6, C7, C8, T1 Recording LeftVocal Cord RLN Recording Right Vocal Cord RLN Recording Right APB-ADMC6, C7, C8, T1 Recording Right Trapezius C3, C4 Recording Right DeltoidC5, C6 Recording Right Flexor Carpi Radialis C6, C7, C8 Recording RightTibialis Anterior L4, L5

TABLE 5 Transcutaneous Nerve Root Stimulation Electrode Type ElectrodePlacement Spinal Level Ground Hip — Anode Mid-back — Stimulation L1-L2cathode — Stimulation Umbilicus anode — Recording Left Adductor MagnusL2, L3, L4 Recording Left Vastus Lateralis L3, L4 Recording LeftTibialis Anterior L4, L5 Recording Left Biceps Femoris L5, S1, S2Recording Right Adductor Magnus L2, L3, L4 Recording Right VastusLateralis L3, L4 Recording Right Vastus Medialis L2, L3, L4 RecordingRight Tibialis Anterior L4, L5 Recording Right Biceps Femoris L5, S1, S2

TABLE 6 SSEP Electrode Type Electrode Placement Spinal Level GroundShoulder — Stimulation Left Post Tibial Nerve — Stimulation Left UlnarNerve — Stimulation Right Post Tibial Nerve — Stimulation Right UlnarNerve — Recording Left Popliteal Fossa — Recording Left Erb's Point —Recording Left Scalp Cp3 — Recording Right Popliteal Fossa — RecordingRight Erb's Point — Recording Right Scalp Cp4 — Recording Center ScalpFpz — Recording Center Scalp Cz — Recording Center Cervical Spine —

The patient module 14 is configured such that the system 10 may conductan impedance test under the direction of the control unit 12 of allelectrodes once the system is set up and the electrode harness isconnected and applied to the patient. After choosing the appropriatespinal site upon program startup (described below), the user is directedto an electrode test. FIGS. 14A-14B illustrate, by way of example only,a graphical implementation capturing the features of an electrodetest[s] as implemented on an electrode test screen 104. The electrodetest screen 104 includes a human figure depiction with positionedelectrodes 108. A harness indicator 109 displays which harness is inuse. For each electrode on the harness 80 in use there is a channelbutton 110. This includes the common 25 and anode 23 electrodes whichare both independently checked for impedance. To accomplish this, theanode 23 and common 25 are both provided as dual electrodes. At leastone of the anode leads on the anode electrode is reversible. During theimpedance check, the reversible anode lead switches to a cathode suchthat the impedance between the leads can be measured. When the impedancetest is complete, the reversible lead switches back to an anode. Thechannel button 110 may be labeled with the muscle or coverage area ofthe corresponding electrode. Selecting the channel button 110 willdisable the channel. Disabled channels will not be tested for impedanceand they will not be monitored for responses or errors unlessreactivated. Upon selection of a start button 106 (“Run ElectrodeTest”), the system tests each electrode individually to determine theimpedance value. If the impedance is determined to be within acceptablelimits, the channel button 110 and electrode depiction on the human FIG.108 turn green. If the impedance value for any electrode is notdetermined to be acceptable, the associated channel button 110 andelectrode depiction turn red, alerting the user. Once the test iscomplete, selecting the “Accept” button 112 will open the mainmonitoring screen 200 of the system 10.

The system 10 may utilize various stimulation accessories to deliverstimulation signals to a stimulation target site such as over thepatient's conus medullaris, a hole formed or being formed in a pedicle,and/or tissue surrounding an access corridor. FIGS. 15-17 illustrate anexample embodiment of a stimulation accessory in the form of astimulation clip 18 that permits the system 10 to deliver stimulationsignals through various surgical instruments already used during thesurgical procedure. By way of example only, the coupling device 18 mayconnect the system 10 with instruments including, but not necessarilylimited to, a pedicle access needle 26, a tap 28, dilator 30, tissueretractor 32, and k-wire 27. The stimulation clip 18 utilizes aspring-loaded plunger 128 to hold the surgical tool and transmit thestimulation signal thereto. The plunger 128 is composed of a conductivematerial such as metal. A nonconductive housing 130 partially encasesthe plunger 128 about its center. Extending from the housing 130 is anendplate 132 that hooks the surgical instrument. A spring (not shown) isdisposed within the housing 130 such that in a natural or “closed”state, the plunger 128 is situated in close proximity to the endplate132. Exerting a compressive force on the spring (such as by pulling onthe thumb grip 134) causes a gap between the end plate 132 and theplunger 128 to widen to an “open” position (shown in FIGS. 15-17 therebyallowing insertion of a surgical tool between the endplate 132 andplunger 128. Releasing the thumb grip 134 allows the spring to return toa “closed” position, causing the plunger 132 to move laterally backtowards the endplate such that a force is exerted upon the surgicalinstrument and thereby holding it in place between the endplate 132 andthe plunger 128. The clip 18 further includes a button module 129containing an activation button 131 for initiating stimulation. Thebutton module 129 is set apart from the body of the clip 18 and they arelinked by an integrated wire. An accessory port 133 is located next tothe button 131 on the button module 129, thus minimizing the number ofwires connecting back to the patient module 14 and outside the sterilefield. Clip 18 is equipped with three LEDs 135, 137, and 139. LED 135 isassociated with the accessory port 133 and LED 137 is associated withthe clip 18 to indicate which of the two is stimulating. The LEDs 137and 137 may appear purple when stimulation is active. When a stimulationresult is determined, the associated LED 135 or 137 may appear eitherred (if the result meets a predetermined potentially unsafe value),green (if the result meets a predetermined safe value), or yellow (ifthe result is in between the safe and potentially unsafe values). Athird LED 139 is contained within the thumb grip 134, which will appearred, yellow, or green depending on the threshold result. The clip 18connects to one of the accessory ports 62 on the patient module 14 via aconnector 136. The connector 136 includes an identification signal thatidentifies it to the patient module.

As mentioned above, the system 10 may include a secondary display, suchas for example only, the secondary display 46 illustrated in FIGS.18-19. The secondary display 46 may be configured to display some or allof the information provided on main display 34. The informationdisplayed to the user on the secondary display 34 may include, but isnot necessarily limited to, alpha-numeric and/or graphical informationregarding any of the selected function modes (e.g. Twitch Test, Free-RunEMG, Basic Stimulated EMG, Dynamic Stimulated EMG, XLIF, MEP Manual, MEPAutomatic, TCNR Alert, TCNR Threshold, SSEP Manual, SSEP Automatic, andsurgical correction planning and assessment), attached accessories (e.g.stimulation probe 16, stimulation clip 18, tilt sensor 54), electrodeharness or harnesses attached, impedance test results, myotome/EMGlevels, stimulation levels, history reports, selected parameters, testresults, etc . . . In one embodiment, secondary display 46 may beconfigured to receive user input in addition to its display function.The secondary display 46 can thus be used as an alternate control pointfor the system 10. The control unit 12 and secondary display 46 may belinked such that input may be received on from one display withoutchanging the output shown on the other display. This would allow thesurgeon to maintain focus on the patient and test results while stillallowing other members of the OR staff to manipulate the system 10 forvarious purposes (e.g. inputting annotations, viewing history, etc. . .. ). The secondary display 46 may be battery powered. Advantageously,the secondary display 46 may be positioned inside the sterile field aswell as outside the sterile field. For positioning within the sterilefield a disposable sterile case 47 may be provided to house the display.Alternatively, the display 46 may be sterile bagged. Both the sterilecase 47 and the secondary display 46 may be mounted to a pole, bedframe, light fixture, or other apparatus found near and/or in thesurgical field. It is further contemplated that multiple secondarydisplays 46 may be linked to the control unit 12. This may effectivelydistribute neurophysiology information and control throughout theoperating room. By way of example, a secondary display 46 may also beprovided for the anesthesiologist. This may be particularly useful inproviding the anesthesiologist with results from the Twitch Test andproviding reminders about the use of paralytics, which may adverselyaffect the accuracy of the system 10. Wired or wireless technology maybe utilized to link the secondary display 46 to the control unit 12.

Having described an example embodiment of the system 10 and the hardwarecomponents that comprise it, the neurophysiological functionality andmethodology of the system 10 will now be described in further detail.Various parameters and configurations of the system 10 may depend uponthe target location (i.e. spinal region) of the surgical procedureand/or user preference. In one embodiment, upon starting the system 10the software will open to a startup screen, illustrated by way ofexample only, in FIG. 20. The startup screen includes a profileselection window 160 from which the user may select from one of thestandard profiles (e.g. “Standard Cervical,” “Standard Thoracolumbar,”and “Standard Lumbar”) or any custom profiles that have been previouslysaved to the system. Profiles may be arranged for selection,alphabetically, by spinal region, or by other suitable criteria.Profiles may be saved to the control unit hard drive or to a portablememory device, such as for example, a USB memory drive, or on a webserver.

Selecting a profile configures the system 10 to the parameters assignedfor the selected profile (standard or custom). The availability ofdifferent function modes may depend upon the profile selected. By way ofexample only, selecting the cervical and thoracolumbar spinal regionsmay automatically configure the options to allow selection of the TwitchTest, SSEP Manual, SSEP Automatic, Basic Stimulated EMG, DynamicStimulated EMG, XLIF, MEP Manual, MEP Automatic, Free-Run EMG modes,while selecting the lumbar region may automatically configure theoptions to allow selection of the Twitch Test, Basic, Difference, andDynamic Stimulated EMG Tests, XLIF®, and Nerve Retractor modes. Defaultparameters associated with the various function modes may also depend onthe profile selected, for example, the characteristics of thestimulation signal delivered by the system 10 may vary depending on theprofile. By way of example, the stimulation signal utilized for theStimulated EMG modes may be configured differently when a lumbar profileis selected versus when one of a thoracolumbar profile and a cervicalprofile.

As previously described above, each of the hardware components includesan identification tag that allows the control unit 12 to determine whichdevices are hooked up and ready for operation. In one embodiment,profiles may only be available for selection if the appropriate devices(e.g. proper electrode harness 80 and stimulation accessories) areconnected and/or ready for operation. Alternatively, the software couldbypass the startup screen and jump straight to one of the functionalmodes based on the accessories and/or harnesses it knows are plugged in.The ability to select a profile based on standard parameters, andespecially on customized preferences, may save significant time at thebeginning of a procedure and provides for monitoring availability rightfrom the start. Moving on from the startup screen, the software advancesdirectly to an electrode test screen and impedance tests, which areperformed on every electrode as discussed above. When an acceptableimpedance test has been completed, the system 10 is ready to beginmonitoring and the software advances to a monitoring screen from whichthe neurophysiological monitoring functions of the system 10 areperformed.

The information displayed on the monitoring screen may include, but isnot necessarily limited to, alpha-numeric and/or graphical informationregarding any of the functional modes (e.g. Twitch Test, Free-Run EMG,Basic Stimulated EMG, Dynamic Stimulated EMG, XLIF, MEP Manual, MEPAutomatic, TCNR Alert, TCNR Threshold, SSEP Manual, SSEP Automatic, andsurgical correction planning and assessment), attached accessories (e.g.stimulation probe 16, stimulation clip 18, tilt sensor 54), electrodeharness or harnesses attached, impedance test results, myotome/EMGlevels, stimulation levels, history reports, selected parameters, testresults, etc. . . . In one embodiment, set forth by way of example only,this information displayed on a main monitoring screen may include, butis not necessarily limited to, the following components as set forth inTable 8:

TABLE 8 Screen Component Description Patient Image/ An image of thehuman body or relevant portion thereof showing the Electrode layoutelectrode placement on the body, with labeled channel number tabs oneach side (1-4 on the left and right). Left and right labels will showthe patient orientation. The channel number tabs may be highlighted orcolored depending on the specific function being performed. Myotome &Level A label to indicate the Myotome name and corresponding SpinalNames Level(s) associated with the channel of interest. Test Menu Ahideable menu bar for selecting between the available functional modes.Device Bar A hideable bar displaying icons and/or names of devicesconnected to the patient module. Display Area Shows procedure-specificinformation including stimulation results. Color Indication Enhancesstimulation results with a color display of green, yellow, or redcorresponding to the relative safety level determined by the system.Stimulation Bar A graphical stimulation indicator depicting the presentstimulation status (i.e. on or off and stimulation current level), aswell as providing for starting and stopping stimulation Event Bar Ahideable bar that shows the last up to a selected number of previousstimulation results, provides for annotation of results, and a chatdialogue box for communicating with remote participants. EMG waveformsEMG waveforms may be optionally displayed on screen along with thestimulation results.

From a profile setting window 160, custom profiles can be created andsaved. Beginning with one of the standard profiles, parameters may bealtered by selecting one of the various buttons and making the changesuntil the desired parameters are set. By way of example only, profilesmay be generated and saved for particular procedures (e.g. ACDF, XLIF,and decompression), particular individuals, and combinations thereof.Clicking on each button will display the parameter options specific tothe selected button in a parameter window. The parameter options for theTest Selection Window are illustrated by way of example in FIG. 20. Byway of example only, by selecting the Test Selection button, sessiontests may be added and viewing options may be changed. From within thetest selection area, function specific parameters for all available testfunctions (based on site selection, available devices, etc . . . ) maybe accessed and set according to need. One option (not shown) that isavailable for multiple functions under the test selection button is theability to select from three different viewing options. The user maychoose to see results displayed in numeric form, on a body panel, and ona label that reflects the labels associated with each electrode, or anycombination of the three. FIGS. 21-32 illustrate examples of the testselection tab 204 for each of the test functions (e.g. Twitch Test,Basic Stimulated EMG, Dynamic Stimulated EMG, XLIF, TCNR Alert, TCNRThreshold, Free-Run, MEP Manual, MEP Automatic, SSEP Manual, SSEPAutomatic). Profiles may be saved directly on the control unit 12, savedto a portable memory device, or uploaded onto a web-server.

The functions performed by the system 10 may include, but are notnecessarily limited to, Twitch Test, Basic Stimulated EMG, DynamicStimulated EMG, XLIF®, Nerve Retractor, TCNR Alert, TCNR Threshold,Free-run EMG, MEP Manual, MEP Automatic, SSEP Manual, SSEP Automatic,and surgical correction planning and assessment modes, all of which willbe described below. The Twitch Test mode is designed to assess theneuromuscular pathway via the so-called “train-of-four-test” to ensurethe neuromuscular pathway is free from muscle relaxants prior toperforming neurophysiology-based testing, such as bone integrity (e.g.pedicle) testing, nerve detection, and nerve retraction. This isdescribed in greater detail within PCT Patent App. No.PCT/US2005/036089, entitled “System and Methods for Assessing theNeuromuscular Pathway Prior to Nerve Testing,” filed Oct. 7, 2005, theentire contents of which is hereby incorporated by reference as if setforth fully herein. The Basic Stimulated EMG Dynamic Stimulated EMGtests are designed to assess the integrity of bone (e.g. pedicle) duringall aspects of pilot hole formation (e.g., via an awl), pilot holepreparation (e.g. via a tap), and screw introduction (during and after).These modes are described in greater detail in PCT Patent App. No.PCT/US02/35047 entitled “System and Methods for Performing PercutaneousPedicle Integrity Assessments,” filed on Oct. 30, 2002, and PCT PatentApp. No. PCT/US2004/025550, entitled “System and Methods for PerformingDynamic Pedicle Integrity Assessments,” filed on Aug. 5, 2004 the entirecontents of which are both hereby incorporated by reference as if setforth fully herein. The XLIF mode is designed to detect the presence ofnerves during the use of the various surgical access instruments of thesystem 10, including the pedicle access needle 26, k-wire 42, dilator44, and retractor assembly 70. This mode is described in greater detailwithin PCT Patent App. No. PCT/US2002/22247, entitled “System andMethods for Determining Nerve Proximity, Direction, and Pathology DuringSurgery,” filed on Jul. 11, 2002, the entire contents of which is herebyincorporated by reference as if set forth fully herein. The NerveRetractor mode is designed to assess the health or pathology of a nervebefore, during, and after retraction of the nerve during a surgicalprocedure. This mode is described in greater detail within PCT PatentApp. No. PCT/US2002/30617, entitled “System and Methods for PerformingSurgical Procedures and Assessments,” filed on Sep. 25, 2002, the entirecontents of which are hereby incorporated by reference as if set forthfully herein. The MEP Manual and Automatic modes are designed to testthe motor pathway to detect potential damage to the spinal cord bystimulating the motor cortex in the brain and recording the resultingEMG response of various muscles in the upper and lower extremities. TheMEP Manual and Automatic modes are described in greater detail withinPCT Patent App. No. PCT/US2006/003966, entitled “System and Methods forPerforming Neurophysiologic Assessments During Spine Surgery,” filed onFeb. 2, 2006, the entire contents of which is hereby incorporated byreference as if set forth fully herein. The TCNR Alert and TCNRThreshold Modes are designed to test potential damage to the lower motorpathway by stimulating trans-abdominally and recording the resulting EMGresponse of various muscles. The TCNR Alert and Threshold modes aredescribed in greater detail within PCT Patent App. No. PCT/US2014/64449,entitled “Systems and Methods for Performing Neurophysiologic MonitoringDuring Spine Surgery,” filed on Nov. 6, 2014, the entire contents ofwhich is hereby incorporated by reference as if set forth fully herein.The SSEP Manual and SSEP Automatic modes are designed to test thesensory pathway to detect potential damage to the spinal cord bystimulating peripheral nerves inferior to the target spinal level andrecording the action potentials superior to the spinal level. The SSEPManual and SSEP Automatic modes are described in greater detail withinPCT Patent App. No. PCT/US2009/05650, entitled “NeurophysiologicMonitoring System and Related Methods,” filed on Oct. 15, 2009, theentire contents of which is hereby incorporated by reference as if setforth fully herein. The surgical correction planning and assessmentmodes are described in greater detail within PCT Patent Application No.PCT/US2014/059974, entitled “Systems for Planning, Performing, andAssessing Spinal Correction during Spine Surgery”, the entire contentsof which is hereby incorporated by reference as if set forth fullyherein. These functions will be explained now in brief detail.

The system 10 performs neuromuscular pathway (NMP) assessments, viaTwitch Test mode, by electrically stimulating a peripheral nerve(preferably the Peroneal Nerve for lumbar and thoracolumbar applicationsand the Median Nerve for cervical applications) via stimulationelectrodes 22 contained in the applicable electrode harness and placedon the skin over the nerve or by direct stimulation of a spinal nerveusing a surgical accessory such as the probe 116. Evoked responses fromthe muscles innervated by the stimulated nerve are detected andrecorded, the results of which are analyzed and a relationship betweenat least two responses or a stimulation signal and a response isidentified. The identified relationship provides an indication of thecurrent state of the NMP. The identified relationship may include, butis not necessarily limited to, one or more of magnitude ratios betweenmultiple evoked responses and the presence or absence of an evokedresponse relative to a given stimulation signal or signals. Withreference to FIG. 21, details of the test indicating the state of theNMP and the relative safety of continuing on with nerve testing areconveyed to the surgeon via GUI display 34. On the monitoring screen 200utilized by the various functions performed by the system 10, functionspecific data is displayed in a center result area 201. The results maybe shown as a numeric value 210, a highlighted label corresponding tothe electrode labels 86, or (in the case of twitch test only) a bargraph of the stimulation results. On one side of center result area 201is a collapsible device menu 202. The device menu displays a graphicrepresentation of each device connected to the patient module 14.Opposite the device menu 202 there is a collapsible test menu 204. Thetest menu 204 highlights each test that is available under the operablesetup profile and may be used to navigate between functions. Acollapsible stimulation bar 206 indicates the current stimulation statusand provides start and stop stimulation buttons (not shown) to activateand control stimulation. The collapsible event bar 208 stores all thestimulation test results obtained throughout a procedure. Clicking on aparticular event will open a note box and annotations may be entered andsaved with the response for later inclusion in a procedure report. Theevent bar 208 also houses a chat box feature when the system 10 isconnected to a remote monitoring system as described above. Within theresult area 202 the twitch test specific results may be displayed.

It should be appreciated that while FIG. 21 depicts the monitoringscreen 200 while the selected function is the Twitch Test, the featuresof monitoring screen 200 apply equally to all the functions.Result-specific data is displayed in a center result area 201. A largecolor saturated numeric value (not shown) is used to show the thresholdresult. Three different options are provided for showing the stimulationresponse level. First, the user can view the waveform. Second, alikeness of the color coded electrode harness label 86 may be shown onthe display. Third, the color coded label 212 may be integrated with abody image. On one side of center result area 201 there is a collapsibledevice menu 202. The device menu displays a graphic representation ofeach device connected to the patient module 14. If a device is selectedfrom the device menu 202, an impedance test may be initiated. Oppositethe device menu 202 there is a collapsible test menu 204. The test menu204 highlights each test that is available under the operable setupprofile and may be used to navigate between functions. A collapsiblestimulation bar 206 indicates the current stimulation status andprovides start and stop stimulation buttons (not shown) to activate andcontrol stimulation. The collapsible event bar 208 stores all thestimulation test results obtained throughout a procedure so that theuser may review the entire case history from the monitoring screen.Clicking on a particular event will open a note box and annotations maybe entered and saved with the response for later inclusion in aprocedure report chronicling all nerve monitoring functions conductedduring the procedure as well as the results of nerve monitoring. In oneembodiment the report may be printed immediately from one or moreprinters located in the operating room or copied to any of a variety ofmemory devices known in the prior art, such as, by way of example only,a floppy disk, and/or USB memory stick. The system 10 may generateeither a full report or a summary report depending on the particularneeds of the user. In one embodiment, the identifiers used to identifythe surgical accessories to the patient module may also be encoded toidentify their lot number or other identifying information. As soon asthe accessory is identified, the lot number may be automatically addedto the report. Alternatively, hand held scanners can be provided andlinked to the control unit 12 or patient module 14. The accessorypackaging may be scanned and again the information may go directly tothe procedure report. The event bar 208 also houses a chat box featurewhen the system 10 is connected to a remote monitoring system to allow auser in the operating room to contemporaneously communicate with aperson performing the associated neuromonitoring in a remote location.

The system 10 may also conduct free-run EMG monitoring while the systemis in any of the modes described herein. Free-run EMG monitoringcontinuously listens for spontaneous muscle activity that might beindicative of potential danger. The system 10 may automatically cycleinto free-run monitoring after 5 seconds of inactivity. Initiating astimulation signal in the selected mode will interrupt the free-runmonitoring until the system 10 has again been inactive for five seconds,at which time the free-run begins again.

The system 10 may test the integrity of pedicle holes (during and/orafter formation) and/or screws (during and/or after introduction) viathe Basic Stimulation EMG and Dynamic Stimulation EMG tests. To performthe Basic Stimulation EMG a test probe 116 is placed in the screw holeprior to screw insertion or placed on the installed screw head and astimulation signal is applied. The insulating character of bone willprevent the stimulation current, up to a certain amplitude, fromcommunicating with the nerve, thus resulting in a relatively highI_(thresh), as determined via the basic threshold hunting algorithmdescribed below. However, in the event the pedicle wall has beenbreached by the screw or tap, the current density in the breach areawill increase to the point that the stimulation current will passthrough to the adjacent nerve roots and they will depolarize at a lowerstimulation current, thus I_(thresh) will be relatively low. The systemdescribed herein may exploit this knowledge to inform the practitionerof the current I_(thresh) of the tested screw to determine if the pilothole or screw has breached the pedicle wall.

In Dynamic Stim EMG mode, test probe 116 may be replaced with a clip 18which may be utilized to couple a surgical tool, such as for example, atap member 28 or a pedicle access needle 26, to the system 10. In thismanner, a stimulation signal may be passed through the surgical tool andpedicle integrity testing can be performed while the tool is in use.Thus, testing may be performed during pilot hole formation by couplingthe access needle 26 to the system 10, and during pilot hole preparationby coupling the tap 28 to the system 10. Likewise, by coupling a pediclescrew to the system 10 (such as via pedicle screw instrumentation),integrity testing may be performed during screw introduction.

In both Basic Stimulation EMG mode and Dynamic Stimulation EMG mode, thesignal characteristics used for testing in the lumbar testing may not beeffective when monitoring in the thoracic and/or cervical levels becauseof the proximity of the spinal cord to thoracic and cervical pedicles.Whereas a breach formed in a pedicle of the lumbar spine results instimulation being applied to a nerve root, a breach in a thoracic orcervical pedicle may result in stimulation of the spinal cord instead,but the spinal cord may not respond to a stimulation signal the same waythe nerve root would. To account for this, the surgical system 10 isequipped to deliver stimulation signals having different characteristicsbased on the region selected. By way of example only, when the lumbarregion is selected, stimulation signals for the stimulated EMG modescomprise single pulse signals. On the other hand, when the thoracic andcervical regions are selected the stimulation signals may be configuredas multipulse signals.

Stimulation results (including but not necessarily limited to at leastone of the numerical I_(thresh) value and color coded safety levelindication) and other relevant data are conveyed to the user on at leastmain display 34, as illustrated in FIGS. 22 and 23. FIG. 22 illustratesthe monitoring screen 200 with the Basic Stimulation EMG test selected.FIG. 23 illustrates the monitoring screen 200 with the DynamicStimulation EMG test selected. In one embodiment of the various screwtest functions (e.g. Basic and Dynamic), green corresponds to athreshold range of greater than 10 milliamps (mA), a yellow correspondsto a stimulation threshold range of 7-10 mA, and a red corresponds to astimulation threshold range of 6 mA or below. EMG channel tabs may beselected via the touch screen display 26 to show the I_(thresh) of thecorresponding nerves. Additionally, the EMG channel possessing thelowest I_(thresh) may be automatically highlighted and/or colored toclearly indicate this fact to the user.

The system 10 may perform nerve proximity testing, via the XLIF mode, toensure safe and reproducible access to surgical target sites. Using thesurgical access components 26-32, the system 10 detects the existence ofneural structures before, during, and after the establishment of anoperative corridor through (or near) any of a variety of tissues havingsuch neural structures which, if contacted or impinged, may otherwiseresult in neural impairment for the patient. The surgical accesscomponents 26-32 are designed to bluntly dissect the tissue between thepatient's skin and the surgical target site. Dilators of increasingdiameter, which are equipped with one or more stimulating electrodes,are advanced towards the target site until a sufficient operatingcorridor is established to advance retractor 32 to the target site. Asthe dilators are advanced to the target site electrical stimulationsignals are emitted via the stimulation electrodes. The stimulationsignal will stimulate nerves in close proximity to the stimulationelectrode and the corresponding EMG response is monitored. As a nervegets closer to the stimulation electrode, the stimulation currentrequired to evoke a muscle response decreases because the resistancecaused by human tissue will decrease, and it will take less current tocause nervous tissue to depolarize. I_(thresh) is calculated, using thebasic threshold hunting algorithm described below, providing a measureof the communication between the stimulation signal and the nerve andthus giving a relative indication of the proximity between accesscomponents and nerves. An example of the monitoring screen 200 with XLIFmode active is depicted in FIG. 24. In a preferred embodiment, a greenor safe level corresponds to a stimulation threshold range of 10milliamps (mA) or greater, a yellow level denotes a stimulationthreshold range of 5-9 mA, and a red level denotes a stimulationthreshold range of 4 mA or below.

In MEP modes, stimulation signals are delivered to the motor cortex viapatient module 14 and resulting responses are detected from variousmuscles in the upper and lower extremities. An increase in I_(thresh)from an earlier test to a later test may indicate a degradation ofspinal cord function. Likewise, the absence of a significant EMGresponse to a given I_(stim) on a channel that had previously reported asignificant response to the same or lesser I_(stim) is also indicativeof a degradation in spinal cord function. These indicators are detectedby the system in the MEP modes and reported to the surgeon. In MEPManual mode, the user selects the stimulation current level and thesystem reports whether or not the stimulation signal evokes asignificant response on each channel. Stimulation results may be shownon the display 34 in the form of “YES” and “NO” responses, or otherequivalent indicia, as depicted in FIG. 25. In MEP Automatic mode thesystem determines the I_(thresh) baseline for each channel correspondingto the various monitored muscles, preferably early in the procedure,using the multi-channel algorithm described. Throughout the proceduresubsequent tests may be conducted to again determine I_(thresh) for eachchannel. The difference between the resulting I_(thresh) values and thecorresponding baseline are computed by the system 10 and comparedagainst predetermined “safe” and “unsafe” difference values. TheI_(thresh), baseline and difference values are displayed to the user,along with any other indicia of the safety level determined (such as ared, yellow, green color code), on the display 34, as illustrated inFIG. 26. Using either mode the surgeon may thus be alerted to potentialcomplications with the spinal cord and any corrective actions deemednecessary may be undertaken at the discretion of the surgeon.

In Transcutaneous Nerve Root Stimulation modes (TCNR), the system 10 iscapable of ascertaining the health and/or status of at-risk nerves alongthe motor neural pathway superior and inferior to the surgical sitebefore, during, and/or after the creation of the operative corridor tothe surgical target site. To accomplish this, stimulation electrodes 22may be placed on the skin over the desired spinal nerve roots (such asby way of example only, the L1 and L2 nerve roots and/or the location ofthe conus medullaris of the patient) and recording electrodes 24 arepositioned on the recording sites (such as, by way of example only, therecording sites set forth above in Table 5). The control unit 12 andpatient module 14 cooperate to transmit electrical stimulation signalsto a stimulating cathode placed posteriorly on the patient's back. Thesestimulation signals cause nerves deep to the stimulating electrode todepolarize, evoking activity from muscles innervated by the nervesbelow. The system 10 detects and records the neuromuscular responses andoptionally analyzes their relationship to the stimulation signal(discussed below). Resulting recording and/or assessment data isconveyed to the user, for example on screen 200. The TCNR testingprovides the ability to verify that the patient is positioned in aneutral way and that no neural structures have been impinged upon afterthe operative corridor has been established. In TCNR Alert mode, theunderlying neurophysiologic principle of operation is to assess thehealth and status of the lower motor neural pathway periodically duringthe surgical procedure via the presence/absence of evoked neuromuscularresponses for each muscle monitored. FIG. 27 depicts an example screendisplay for the Alert mode of the TCNR monitoring function. In TCNRThreshold mode, the underlying neurophysiologic principle is that, inorder to monitor the health of the lumbar motor neural pathway, the usermust be able to determine if the evoked responses are changing withrespect to the stimulation signal. To monitor for this change, baselineTCNR responses are established for all muscles of interest (preferablyprior to surgical manipulation) and then compared to subsequent TCNRresponses periodically throughout the procedure. FIG. 28 depicts anexample screen display for the Threshold mode of the TCNR monitoringfunction.

In the surgical correction planning and assessment mode, the system 10aids the user in planning and assessing the degree to which he/she hasachieved the surgical goals during a spinal procedure. As illustrated,by way of example only, in FIG. 29, there may be included a feature inwhich the system 10 is capable of digitizing implanted screw positions,outputting bend instructions for a rod, and previewing the shape of therod on screen 200. In some implementations, the rod bending instructionsare for a rod shaped to custom-fit within the implanted screw locations.In other implementations, the system 10 is further capable of acceptingcorrection inputs via one or more advanced option features for viewingbend instructions for a rod shaped to fit at locations apart from thoseimplanted screw locations. Installing a rod shaped in this manner couldcorrect a curvature deformity in the patient's spine according to auser's prescribed surgical plan. In some spinal procedures, restoring apatient's spine to a balanced position may be a desired surgicaloutcome. As shown in FIG. 30, there may also be included a feature inwhich the system 10 is configured to receive and assess 1) preoperativespinal parameter inputs; 2) target spinal parameter inputs; 3)intraoperative spinal parameter inputs; and/or postoperative spineparameter inputs via screen 200. Spinal parameters may comprise thepatient's Pelvic Incidence (PI), Pelvic Tilt (PT), Sacral Slope (SS),Lumbar Lordosis (LL), Superior Lumbar Lordosis (⬆LL), Inferior LumbarLordosis (⬇LL), C7 Plumb Line Offset (C7PL), Thoracic Kyphosis (TK), T1Tilt, and Sagittal, Vertical Axis (SVA) measurements. One or more ofthese inputs may be tracked and/or compared against other inputs toassess how the surgical correction is progressing toward a surgical planand utilized to develop/refine an operative plan to achieve the desiredsurgical correction.

In the SSEP modes, the system 10 stimulates peripheral sensory nervesthat exit the spinal cord below the level of surgery and then measuresthe electrical action potential from electrodes located on the nervoussystem superior to the surgical target site. Recording sites below theapplicable target site are also preferably monitored as a positivecontrol measure to ensure variances from normal or expected results arenot due to problems with the stimulation signal delivered (e.g.misplaced stimulation electrode, inadequate stimulation signalparameters, etc.). To accomplish this, stimulation electrodes 22 may beplaced on the skin over the desired peripheral nerve (such as by way ofexample only, the left and right Posterior Tibial nerve and/or the leftand right Ulnar nerve) and recording electrodes 24 are positioned on therecording sites (such as, by way of example only, at least two of the C2vertebra, Cp3 scalp, Cp4 scalp, Erb's point, Popliteal Fossa) andstimulation signals are delivered from the patient module 14.

Damage in the spinal cord may disrupt the transmission of the signal upalong the spinothalamic pathway through the spinal cord resulting in aweakened, delayed, or absent signal at the recording sites superior tothe surgery location (e.g. cortical and subcortical sites). To check forthese occurrences, the system 10 monitors the amplitude and latency ofthe evoked signal response. According to one embodiment, the system 10may perform SSEP in either of two modes: Automatic mode and Manual mode.In SSEP Auto mode, the system 10 compares the difference between theamplitude and latency of the signal response vs. the amplitude andlatency of a baseline signal response. The difference is comparedagainst predetermined “safe” and “unsafe” levels and the results aredisplayed on display 34. According to one embodiment, the system maydetermine safe and unsafe levels based on each of the amplitude andlatency values for each of the cortical and subcortical sitesindividually, for each stimulation channel. That is, if either of thesubcortical and cortical amplitudes decrease by a predetermined level,or either of the subcortical and cortical latency values increase by apredetermined level, the system may issue a warning. By way of example,the alert may comprise a Red, Yellow, Green type warning associated withthe applicable channel wherein Red indicates that at least one of thedetermined values falls within the unsafe level, the color green mayindicate that all of the values fall within the safe level, and thecolor yellow may indicate that at least one of the values falls betweenthe safe and unsafe levels. To generate more information, the system 10may analyze the results in combination. With this information, inaddition to the Red, Yellow, and Green alerts, the system 10 mayindicate possible causes for the results achieved. In SSEP Manual mode,signal response waveforms and amplitude and latency values associatedwith those waveforms are displayed for the user. The user then makes thecomparison between a baseline the signal response.

FIGS. 31-36 are exemplary screen displays of the “SSEP Manual” modeaccording to one embodiment of the neuromonitoring system 10. FIG. 31illustrates an setup screen from which various features and parametersof the SSEP Manual mode may be controlled and/or adjusted by the user asdesired. Using this screen, the user has the opportunity to togglebetween Manual mode and Automatic mode, select a stimulation rate, andchange one or more stimulation settings (e.g. stimulation current, pulsewidth, and polarity) for each stimulation target site (e.g. left ulnarnerve, right ulnar nerve, left tibial nerve, and right tibial nerve). Byway of example only, the user may change one or more stimulationsettings of each peripheral nerve by first selecting one of thestimulation site tabs 264.

Selecting one of the stimulation site tabs 264 will open a controlwindow 265, seen in FIG. 32, from which various parameters of the SSEPmanual test may be adjusted according to user preference. By way ofexample only, FIG. 32 is an illustration of an onscreen display for theSSEP manual test settings of the left ulnar nerve stimulation site. Thehighlighted “Left Ulnar Nerve” stimulation site tab 264 and the pop-upwindow title 266 indicate that adjusting any of the settings will alterthe stimulation signal delivered to the left ulnar nerve. Multipleadjustment buttons are used to set the parameters of the stimulationsignal. According to one example, the stimulation rate may be selectedfrom a range between 2.2 and 6.2 Hz, with a default value of 4.7 Hz. Theamplitude setting may be increased or decreased in increments of 10 mAusing the amplitude selection buttons 270 labeled (by way of exampleonly) “+10” and “−10”. More precise amplitude selections may be made byincreasing or decreasing the amplitude in increments of 1 mA using theamplitude selection buttons 272 labeled (by way of example only) “+1”and “−1”. According to one example, the amplitude may be selected from arange of 1 to 100 mA with a default value of 10 mA. The selectedamplitude setting is displayed in box 274. The pulse width setting maybe increased or decreased in increments of 50 μsec using the widthselection buttons 276 labeled “+50” and “−50”. According to one example,the pulse width may be selected from a range of 50 to 300 μsec, with adefault value of 200 μsec. The precise pulse width setting 278 isindicated in box 278. Polarity controls 280 may be used to set thedesired polarity of the stimulation signal. SSEP stimulation may beinitiated at the selected stimulation settings by pressing the SSEPstimulation start button 284 labeled (by way of example only) “StartStim.” Although stimulation settings adjustments are discussed withrespect to the left ulnar nerve, it will be appreciated that stimulationadjustments may be applied to the other stimulation sites, including butnot limited to the right ulnar nerve, and left and right tibial nerve.Alternatively, as described below, the system 10 may utilize anautomated selection process to quickly determine the optimal stimulationparameters for each stimulation channel.

In order to monitor the health of the spinal cord with SSEP, the usermust be able to determine if the responses to the stimulation signal arechanging. To monitor for this change a baseline is determined,preferably during set-up. This can be accomplished simply by selectingthe “set as baseline” button 298 next to the “start stim” button 284 onthe setting screen illustrated in FIG. 32. Having determined a baselinerecording for each stimulation site, subsequent monitoring may beperformed as desired throughout the procedure and recovery period toobtain updated amplitude and latency measurements.

FIG. 33 depicts an exemplary screen display for Manual mode of the SSEPmonitoring function. A mode indicator tab 290 on the test menu 204indicates that “SSEP Manual” is the selected mode. The center resultarea 201 is divided into four sub areas or channel windows 294, each onededicated to displaying the signal response waveforms for one of thestimulation nerve sites. The channel windows 294 depict informationincluding the nerve stimulation site 295, and waveform waterfalls foreach of the recording locations 291-293. For each stimulated nerve site,the system 10 displays three signal response waveforms, representing themeasurements made at three different recording sites. By way of exampleonly, the three recording sites are a peripheral 291 (from a peripheralnerve proximal to the stimulation nerve), subcortical 292 (spine), andcortical 293 (scalp), as indicated for example in Table 6 above. Eachsection may be associated with a pictorial icon, illustrating theneural/skeletal structure. Although SSEP stimulation and recording isdiscussed with respect to the nerve stimulation site and the recordingsites discussed above, it will be appreciated that SSEP stimulation maybe applied to any number of peripheral sensory nerves and the recordingsites may be located anywhere along the nervous system superior to thespinal level at risk during the procedure.

During SSEP modes (auto and manual), a single waveform response isgenerated for each stimulation signal run (for each stimulationchannel). The waveforms are arranged with stimulation on the extremeleft and time increasing to the right. By way of example, the waveformsare captured in a 100 msec window following stimulation. The stimulationsignal run is comprised of a predefined number of stimulation pulsesfiring at the selected stimulation frequency. By way of example only,the stimulation signal may include up to 300 pulses at a frequency of4.7 Hz. A 100 ms window of data is acquired on each of three SSEPrecording channels: cortical, subcortical, and peripheral. With eachsuccessive stimulation on the same channel during a stimulation run, thethree acquired waveforms are summed and averaged with the priorwaveforms during the same stimulation run for the purpose of filteringout asynchronous events such that only the synchronous evoked responseremains after a sufficient number of pulses. Thus, the final waveformdisplayed by the system 10 represents an averaging of the entire set(e.g. 300) of responses detected.

With each subsequent stimulation run, waveforms are drawn slightly lowereach time, as depicted in FIGS. 33-35, until a total of four waveformsare showing. After more than four stimulation runs, the baselinewaveform is retained, as well as the waveforms from the previous fourstimulation runs. Older waveforms are removed from the waveform display.According to one embodiment, different colors may be used to representthe different waveforms. For example, the baseline waveforms may becolored purple, the last stimulation run may be colored white, thenext-to-last stimulation run may be colored medium gray, and theearliest of the remaining stimulation runs may be colored dark gray.

According to one example, the baseline and the latest waveforms may havemarkers 314, 316 placed indicating latency and amplitude valuesassociated with the waveform. The latency is defined as the time fromstimulation to the first (earliest) marker. There is one “N” 314 and one“P” 316 marker for each waveform. The N marker is defined as the maximumaverage sample value within a window and the P value is defined as theminimum average sample value within the window. The markers may comprisecross consisting of a horizontal and a vertical line in the same coloras the waveform. Associated with each marker is a text label 317indicating the value at the marker. The earlier of the two markers islabeled with the latency (e.g. 22.3 ms). The latter of the two markersis labeled with the amplitude (e.g. 4.2 uV). The amplitude is defined asthe difference in microvolts between average sample values at themarkers. The latency is defined as the time from stimulation to thefirst (earliest) marker. Preferably, the markers are placedautomatically by the system 10 (in both auto and manual modes). Inmanual mode, the user may select to place (and or move) markersmanually.

Further selecting one of the channel windows 294 will zoom in on thewaveforms contained in that window 294. FIG. 36 is an exampleillustration of the zoom view achieved by selecting one of the channelwindows 294. The zoom view includes waveforms 291-293, the baselinewaveform, markers 314 and 316, and controls for moving markers 318 andwaveform scaling 332. Only the latest waveform is shown. The “Set All asBaseline” button 310 will allow the user to set (or change) all threerecorded waveforms as the baselines. Additionally, baselines may be set(or changed) individually by pressing the individual “Set as Baseline”buttons 312. Furthermore, the user may also move the N marker 314 and Pmarkers 316 to establish new measurement points if desired. Directioncontrol arrows 318 may be selected to move the N and P markers to thedesired new locations. Alternatively, the user may touch and drag themarker 314, 316 to the new location. Utilizing the waveform controls 332the user may zoom in and out on the recorded waveform.

Referencing FIGS. 37-40, Automatic SSEP mode functions similar to ManualSSEP mode except that the system 10 determines the amplitude and latencyvalues and alerts the user if the values deviate. FIG. 37 shows, by wayof example only, an exemplary setup screen for the SSEP Automatic mode.In similar fashion to the setup screen previously described for the SSEPManual mode, the user may toggle between Manual mode and Automatic mode,select a stimulation rate, and change one or more stimulation settings.By way of example only, the user may change one or more stimulationsettings of each peripheral nerve by first selecting one of thestimulation site tabs 264, as described above with reference to Manualmode and FIG. 18. According to one example, the stimulation rate may beselected from a range between 2.2 and 6.2 Hz, with a default value of4.7 Hz, the amplitude may be selected from a range of 1 to 100 mA, witha default value of 10 mA, the pulse width may be selected from a rangeof 50 to 300 μsec, with a default value of 200 μsec.

In Automatic mode, the system 10 also includes a timer function whichcan be controlled from the setup screen. Using the timer drop down menu326, the user may set and/or change a time interval for the timerapplication. There are two separate options of the timer function: (1)an automatic stimulation on time out which can be selected by pressingthe auto start button 322 labeled (by way of example only) “Auto StartStim when timed out”; and (2) a prompted stimulation reminder on timeout which can be selected by pressing the prompt stimulation button 324labeled (by way of example only) “Prompt Stim when timed out”. Aftereach SSEP monitoring episode, the system 10 will initiate a timercorresponding to the selected time interval and, when the time haselapsed, the system will either automatically perform the SSEPstimulation or a stimulation reminder will be activated, depending onthe selected option. The stimulation reminder may include, by way ofexample only, any one of, or combination of, an audible tone, voicerecording, screen flash, pop up window, scrolling message, or any othersuch alert to remind the user to test SSEP again. It is alsocontemplated that the timer function described may be implemented inSSEP Manual mode.

FIGS. 38-40 depict exemplary onscreen displays for Automatic mode of theSSEP function. According to one embodiment, the user may select to viewa screen with only alpha-numeric information (FIG. 39) and one withalpha-numeric information and recorded waveforms (FIG. 38). A modeindicator tab 290 indicates that “SSEP Auto” is the selected mode. Awaveform selection tab 330 allows the user to select whether waveformswill be displayed with the alpha-numeric results. In similar fashion tothe onscreen displays previously described for the SSEP Manual mode, thesystem 10 includes a channel window 294 for each nerve stimulation site.The channel window 294 may display information including the nervestimulation site 295, waveform recordings, and associated recordinglocations 291-293 (peripheral, sub cortical, and cortical) and thepercentage change between the baseline and amplitude measurements andthe baseline and latency measurements. By way of example only, eachchannel window 294 may optionally also show the baseline waveform andlatest waveform for each recording site. In the event the system 10detects a significant decrease in amplitude or an increase in latency,the associated window may preferably be highlighted with a predeterminedcolor (e.g. red) to indicate the potential danger to the surgeon.Preferably, the stimulation results are displayed to the surgeon alongwith a color code so that the user may easily comprehend the danger andcorrective measures may be taken to avoid or mitigate such danger. Thismay for example, more readily permit SSEP monitoring results to beinterpreted by the surgeon or assistant without requiring dedicatedneuromonitoring personnel. By way of example only, red is used when thedecrease in amplitude or increase in latency is within a predeterminedunsafe level. Green indicates that the measured increase or decrease iswithin a predetermined safe level. Yellow is used for measurements thatare between the predetermined unsafe and safe levels. By way of exampleonly, the system 10 may also notify the user of potential danger throughthe use of a warning message 334. Although the warning message is in theform of a pop-up window, it will be appreciated that the warning may becommunicated to the user by any one of, or combination of, an audibletone, voice recording, screen flash, scrolling message, or any othersuch alert to notify the user of potential danger.

With reference to FIG. 40, at any time during the procedure, a priorstimulation run may be selected for review. This may be accomplished by,for example, by opening the event bar 208 and selecting the desiredevent. Details from the event are shown with the historical detailsdenoted on the right side of the menu screen 302 and waveforms shown inthe center result screen. Again, the user may choose to reset baselinesfor one or more nerve stimulation sites by pressing the appropriate “SetAs Baseline” button 306. In the example shown, the system 10 illustratesthe waveform history at the 07:51 minute mark which is denoted on theright side of the menu screen 302. Prior waveform histories are saved bythe surgical system 10 and stored in the waveform history toolbar 304.The describe only in relation to the SSEP Auto function it will beappreciated that the same features may be accessed from SSEP Manualmode, the user may choose to set a recorded stimulation measurement asthe baseline for each nerve stimulation site by pressing the “Set AsBaseline” button 306. By way of example only, the system 10 will informthe user if the applicable event is already the current baseline with a“Current Baseline” notification 308.

In addition to alerting the user to any changes in the amplitude and/orlatency of the SSEP signal response, it is further contemplated that theneuromonitoring system 10 may assess the data from all the recordingsites to interpret possible causes for changes in the SSEP response.Based on that information, the program may suggest potential reasons forthe change. Furthermore, it may suggest potential actions to be taken toavoid danger. It is still further contemplated that the neurophysiologysystem 10 may be communicatively linked with other equipment in theoperating room, such as for example, anesthesia monitoring equipment.Data from this other equipment may be considered by the program togenerate more accuracy and or better suggestions.

By way of example only, Table 9 illustrates the SSEP illustrates variouswarnings that may be associated with particular SSEP results and resultcombinations from cortical, subcortical, and peripheral responses, andshown to the user. For example, if in response to stimulation of theleft ulnar nerve, the peripheral response from Erb's Point showed nochange in amplitude or latency, the subcortical response showed adecrease in amplitude, and the cortical response showed a decrease inamplitude, the event box 206 (shown in FIG. 39) would show either ayellow or a red indicator as well as the text “Possible mechanicalinsult. Possible spinal cord ischemia.” By way of another example, ifthere is a decreased amplitude or absent response in all peripheral,subcortical, and cortical recording sites, the system may show a “CheckElectrode” warning 332 (FIG. 40). With this date, and the particularcircumstances leading to the result (e.g. what surgical maneuverresulted in the warning, etc.) the user may be better equipped todetermine the most prudent course of action.

TABLE 9 Audio-visual Neurophysiologic Event Alert (Color) SSEP ExpertText Cortical amplitude decrease: Green No Warning 0-25% from baselineCortical amplitude decrease: Yellow “Some anesthetic agents may reducethe 26-49% from baseline cortical response amplitude.” Corticalamplitude decrease: Red “Some anesthetic agents may reduce the 50%-99%from baseline cortical response amplitude.” Cortical amplitude decrease:Red “Possible cortical ischemia.” 100% from baseline Cortical latencyincrease: Green No Warning 0-5% from baseline Cortical latency increase:Yellow “Some anesthetic agents may increase the 6-9% from baselinecortical response latency. Possible cortical ischemia.” Cortical latencyincrease: Red “Some anesthetic agents may increase the 10% or greaterfrom baseline cortical response latency. Possible cortical ischemia.”Cortical response absent: Red “Some anesthetic agents may cause thecortical response to be absent. Possible cortical ischemia.” Subcorticalamplitude decrease: Green No Warning 0%-25% from baseline Subcorticalamplitude decrease: Yellow “Possible muscle activity artifact. Possible25%-49% from baseline cervical recording electrode issue.” Subcorticalamplitude decrease: Red “Possible muscle activity artifact. Possible50-99% from baseline or absent cervical recording issue.” 50% amplitudedecrease, 10% latency Red “Possible mechanical insult. Possible increasein both cortical and subcortical spinal cord ischemia.” responses, orabsence in both cortical and subcortical responses: Peripheral amplitudedecrease: Red “Possible peripheral recording electrode greater than 50%or absent issue.” Peripheral (Erb's Point) amplitude Green No Warning(left or right) decrease: 0-25% from baseline Peripheral (Erb's Point)amplitude Yellow “Possible peripheral recording electrode decrease:issue (Left Erb's Point).” 26-49% from baseline “Possible peripheralrecording electrode issue (Right Erb's Point).” Peripheral (Erb's Point)amplitude Red “Possible peripheral recording electrode decrease: issue(Left Erb's Point).” 50%-100% from baseline “Possible peripheralrecording electrode issue (Right Erb's Point).” Peripheral (PoplitealFossa amplitude Green No Warning (left or right) decrease: 0-25% frombaseline Peripheral (Popliteal Fossa) amplitude Yellow “Possibleperipheral recording electrode decrease: issue (Left Popliteal Fossa).”26-49% from baseline “Possible peripheral recording electrode issue(Right Popliteal Fossa).” Peripheral (Popliteal Fossa) amplitude Red“Possible peripheral recording electrode decrease: issue (Left PoplitealFossa).” 50%-100% from baseline “Possible peripheral recording electrodeissue (Right Popliteal Fossa).” Peripheral (Erb's Point) latencyincrease: Green No Warning (left or right) 0-5% from baseline Peripheral(Erb's Point) latency increase: Yellow No Warning (left or right) 6-9%from baseline Peripheral (Erb's Point) latency increase: Red No Warning(left or right) 10% or greater from baseline Peripheral (PoplitealFossa) latency Green No Warning (left or right increase: 0-5% frombaseline Peripheral (Popliteal Fossa) latency Yellow No Warning (left orright) increase: 6-9% from baseline Peripheral (Popliteal Fossa) latencyRed No Warning (left or right) increase: 10% or greater from baselinePeripheral (Popliteal Fossa) and Green Possible muscle activityartifact. Possible subcortical amplitude decrease: cervical recordingelectrode issue. (left or 0-25% from baseline right) Peripheral(Popliteal Fossa) and Yellow/Red “Possible cervical muscle activityartifact. subcortical amplitude decrease: Possible cervical recordingelectrode issue. 26%-100% from baseline Possible muscle activityartifact (posterior tibial nerve).” (left or right) Peripheral (Erb'sPoint) and subcortical Green “Possible muscle activity artifact.amplitude decrease: Possible cervical recording electrode 0-25% frombaseline issue.” (left or right) Peripheral (Erb's Point) andsubcortical Yellow/Red “Possible cervical muscle activity artifact.amplitude decrease: Possible cervical recording electrode issue. 26-99%from baseline Possible muscle activity artifact (median nerve).” (leftor right) Decreased amplitude or absent response Yellow/Red “Possiblestimulating electrode issue. (left in all, peripheral (left Erb'spoint), wrist).” subcortical, and cortical: Decreased amplitude orabsent in all, Yellow/Red “Possible stimulating electrode issue (rightperipheral (right Erb's point), subcortical, wrist).” and cortical:Decreased amplitude or absent response Yellow/Red “Possible stimulatingelectrode issue (left in all peripheral (left Popliteal Fossa), ankle).”subcortical, and cortical: Decreased amplitude or absent responseYellow/Red Possible stimulating electrode issue (right in all peripheral(right Popliteal Fossa), ankle) subcortical, and cortical Increasedlatency or decreased amplitude Yellow/Red “Possible systemic change(hypotension, in all, peripheral, subcortical, and hypothermia,hyperthermia). Possible cortical: peripheral nerve ischemia.” (left orright) (posterior tibial or ulnar nerve)

According to some implementations, the various warnings that may beassociated with particular SSEP results may be displayed on ananatomical diagram of the body as shown in FIG. 41. Alerts may bedisplayed on the diagram to represent the peripheral and spinal segmentregions as set forth, by way of example only, in Table 10. According toone or more embodiments, it may be desirable to record from all fourstimulation sites (e.g. Left Posterior Tibial Nerve, Right PosteriorTibial Nerve, Left Ulnar Nerve, and Right Ulnar nerve) from just tworecording sites (by way of example, one subcortical recording locationreferenced to one cortical recording location). Such a configurationpotentially alleviates the complexity associated with the differentalert criteria required when managing peripheral, subcortical, andcortical responses simultaneously. By having all four stimulation sitesuse the same recording location, the user may be provided withinformation as to the health and status of the spinal cord withoutdifferentiating between electrodes. This facilitates the ability totroubleshoot technical issues. For example, noise caused by a dislodgedrecording electrode can be limited to one recording electrode to beexamined. This also facilitates taking into account anesthetic effectson SSEP signals. For example, if there is a change to the anestheticregime that affects the amplitude and/or latency of any responses, allfour SSEP responses will change by the same amount because all responsesare equally affected by anesthesia. This provides a direct method fordetermining the overall impact the anesthetic regime is having on theSSEP responses.

TABLE 10 Change from Baseline Anatomy Highlight Visual Alert/Audio AlertAmplitude >50% and upper limb green limb No message, tone associatedLatency <10% cervical spine green spine with green thoracic spine greenspine lower limb green limb No message, no tone electrode Amplitude ≤50%or upper limb yellow limb “Significant change detected. Latency ≥10%cervical spine yellow spine Notify Surgeon and thoracic spine yellowspine Anesthesiologist.” Tone associated with yellow lower limb yellowlimb “Significant change detected. electrode Notify Surgeon andAnesthesiologist.” Tone associated with yellow

In accordance with the present disclosure, there are provided one ormore algorithms executable on the control unit 14 of the system 10 thatquickly enable the acquisition of a well-resolved SSEP waveform with aminimum number of stimulations, without the need for highly trainedpersonal to be present to acquire or interpret the waveform. Thesealgorithms may be used alone or in combination with one another torefine stimulation parameters until a combination resulting in the mostdesirable result is achieved. It is to be appreciated while thealgorithms described below are described with respect to somatosensoryevoked potentials (SSEP), it is equally applicable to allneurophysiologic modalities, including but not limited to motor evokedpotential testing (MEP) and transcutaneous nerve root (TCNR) testing. Itwill also be appreciated that the algorithms described below areapplicable to not just spine procedures but any procedure in which oneor more aspects of the nervous system are at risk of permanent ortransient injury or damage.

According to a broad aspect of the present invention, there is provideda baseline hunting algorithm that quickly ascertains a noise rejectionthreshold (NRT) and automatically optimizes baseline stimulationparameters for use in subsequent SSEP testing performed during thesurgical procedure. According to one implementation, the baselinehunting algorithm may include the following steps: (a) establish arejection threshold to minimize the number of responses needed and theamount of noise allowed into the average; (b) use a noise floor windowto isolate the white background noise level for comparison to theneurophysiological response; (c) analyze the stability of waveformmarkers to ensure they are reproducible amongst consecutivestimulations; and (d) store stimulation parameters for all stimulationsites when such stimulation parameters elicit clinically significantSSEP responses. Optionally, after the stimulation parameters areobtained for one stimulation site, the algorithm may optionally increasethe stimulation frequency for the remaining stimulation sites toidentify the proper stimulation parameters as quickly as possible. Onceoptimized baseline stimulation parameters are obtained, baseline SSEPrecordings may be obtained and monitored during the procedure forsignificant changes to those baseline recordings.

In general, minimizing the noise allowed into an SSEP average willreduce the number of responses required for a well-resolved response. Inaccordance with the present disclosure, a noise rejection thresholdparameter is used to minimize the number of responses added to theaverage that have excessive interference. Referring now to FIG. 42, atstep 300, the stimulating hunting algorithm process is started. Thealgorithm will first hunt for an optimal noise rejection threshold toensure that a significant number of stimulations are being allowed intothe average (step 302). According to one embodiment, the system 10analyzes how many stimulations are rejected as falling outside of therejection threshold level. If the rejection rate is more than 50%, thesystem 10 increases the noise rejection threshold incrementally (e.g.,+10 μV) shown as step 304 until the rejection rate is less than 50%. Ifthe noise rejection threshold reaches a maximum (e.g., 250 μV) withoutreaching a less than 50% rejection rate, then the user is alerted tofind the source of the interference before proceeding with SSEPs (e.g. avisual alert on the GUI screen “check electrode connection”). Once therejection rate is less than 50%, the system 10 sets that rejectionthreshold level as the noise rejection level (i.e. noise floor) and thealgorithm proceeds to refining the stimulation parameters as set forthbelow.

Once the noise rejection threshold has been identified, the stimulationcontinues and the algorithm proceeds to evaluate the stability of theSSEP waveforms. Stability of the SSEP waveforms may be evaluated as thedegree to which an SSEP response moves or shifts each time it isrefreshed on the graphical user interface. A stable response may be onethat does not move/shift an appreciable amount between stimulations andan unstable response may be one that does move/shift an appreciableamount between stimulations. By way of example, the system 10 maymeasure stability by tracking latency over time. This may beaccomplished, for example, via a pixel analysis to determine when thewaveform is settled down into a consistent averaged response. At step306, the system 10 continues to direct stimulation and measures thestability of the SSEP waveforms. According to one or moreimplementations, a waveform may be deemed “stable” when its latencymarkers does not vary more than 2 msec in between responses tosuccessive stimulations.

Once the waveform is deemed stabilized such that N and P markers 314,316 can be placed, the amplitude of the waveform is compared to thenoise floor (defined as the noise rejection threshold obtained at step302). At step 308, the system 10 compares the V_(pp) of the response tothe V_(pp) of the noise floor to make sure that the responsesufficiently greater than the noise floor to be deemed clinicallysignificant. By way of example, a response may be deemed clinicallysignificant when its amplitude is 2.2 times the amplitude of the noisefloor. If the response is not sufficiently greater than the noise floor,the stimulation current is increased until a clinically significantresponse is evoked. By way of example, the stimulation intensity may beincreased at step 310 incrementally (e.g. 10 mA) until a clinicallysignificant response is found or until a maximum current is reached(e.g. 100 mA). The stimulation current level that produced thatclinically significant response for that is recorded in memory at step312. This stimulation current will be used for the baseline stimulationand all subsequent stimulations throughout the procedure. Stimulationhunting continues until the optimal parameters have been stored for eachstimulation site. As shown in step 314, according to someimplementations, the stimulation rate may be increased for remainingstimulation sites to make the hunting process as efficient as possible.By way example only, the stimulation rate may be increased to themaximum stimulation rate allowed for the remaining simulation sites thathave not found a response.

After the stimulation intensity has been determined for all stimulationsites, the stored stimulation parameters are used to run the baseline(step 316). As shown in step 318, subsequent stimulations are run duringthe surgical procedure and compared to the baseline to determine ifthere is a significant change in the SSEP responses in one or morechannels. As shown in step 320, SSEP changes are determined to beclinically significant if they exceed a predetermined criteria (e.g., anamplitude decrease of >50% and/or a latency increase of >10% frombaseline). If the answer is yes, (step 322) there is a deemedsignificant change. If the answer is no (step 324), there is nosignificant change. These alerts are displayed to the user, by way ofexample, only, using one of the methods set forth above.

As set forth above, SSEP monitoring involves recording very low levelsignals in the presence of other biologic signals and noise. To overcomethe noise, the evoked response is recorded anywhere from 50 to 500 timesto average out any non-coherent, non-time locked response. The result isa clear waveform representing the desired nerve signal. In prior artsystems, the user of the manually selects the number of times that thesignal will be recorded for the entire procedure. If the noise level ishigh, a high number of samples to be taken. If the noise is low, then alow number of samples is taken. However, the ability to quickly obtainan SSEP result once the waveform is sufficiently well-resolved providesfeedback to the user in a meaningful way. The present disclosuredescribes an algorithmic means of determining the number of evokedresponses to include in the data set.

According to another broad aspect of the present invention, there isprovided an algorithm for determining the number of evoked responses toinclude in a data set based on the variance of V_(pp) amplitudes in thewaveform. The algorithm will stop the averaging process once randombiologic and noise information has been sufficiently averaged out.According to one implementation shown by way of example in FIG. 43, thepositive and negative peaks of the waveform are located and theirrelative amplitude is measured (step 400). The variance of the V_(pp) ofthe waveform is monitored over time as stimulations continue (step 402).When the variance achieves a specified consistency between data sets(for example <5%), the algorithm will automatically stop the SSEP datacollection (step 404). For example, if the data collection is revealinga nerve signal peak-to-peak voltage of 1 μV and the next data set addedto the average changes the amplitude by less than 5%, then the algorithmwould command a stop to the data collection. It is to be appreciatedthat the tolerance for variations in the signal could be setparametrically (e.g. 0.5%, 1%, 5%) as well as the number of datasetsthat arrive within the established average (e.g. 1, 5, 10).

Low amplitude SSEP signals are often recorded in the presence of ACpower line noise (typically 60 Hz or 50 Hz). The available SSEPstimulation rates are calculated to capture alternate positive andnegative phases of the AC power line frequency, maximizing the rejectionof power line noise through cancellation. Very often, when gatheringsweeps to include in an average, a number of sweeps may be excluded froma given average due to amplitude rejection criteria, data communicationinterruption, or other causes. If an unequal number of positive ornegative phase sweeps are included in the average, the final SSEPaverage may retain an undesirably high residual level of line frequencynoise. The larger the discrepancy between the number of positive andnegative phases included in the average, the larger that remainingresidual level will be.

According to another broad aspect of the present invention, there isprovided an algorithm to control the number of positive and negativephase sweeps included in the average. According to some implementations,the system 10 tracks the number of each phase included in the average,and adjust (by adding an appropriate number of positive or negativephase sweeps, whichever is lacking, or by removing an appropriate numberof positive or negative phase sweeps, whichever is in excess) until thenumber of positive and negative phase sweeps in the average are equal.This could be done dynamically during, or at the end of an average.

According to one embodiment, sequential stimulations are tracked throughthe use of an identification tag, and a score of positive phase andnegative phase line frequency components can be maintained. The firmwareand/or the software application can then determine if an unmatched phaseof line frequency noise is in the data set and thereby choose to collectadditional data sets or subtract a data set until the score is matchedfor both phases.

While this invention has been described in terms of a best mode forachieving this invention's objectives, it will be appreciated by thoseskilled in the art that variations may be accomplished in view of theseteachings without deviating from the spirit or scope of the presentinvention. For example, the present invention may be implemented usingany combination of computer programming software, firmware or hardware.As a preparatory step to practicing the invention or constructing anapparatus according to the invention, the computer programming code(whether software or firmware) according to the invention will typicallybe stored in one or more machine readable storage mediums such as fixed(hard) drives, diskettes, optical disks, magnetic tape, semiconductormemories such as ROMs, PROMs, etc., thereby making an article ofmanufacture in accordance with the invention. The article of manufacturecontaining the computer programming code is used by either executing thecode directly from the storage device, by copying the code from thestorage device into another storage device such as a hard disk, RAM,etc. or by transmitting the code on a network for remote execution. Ascan be envisioned by one of skill in the art, many differentcombinations of the above may be used and accordingly the presentinvention is not limited by the specified scope.

While the invention is susceptible to various modifications andalternative forms, specific embodiments thereof have been shown by wayof example in the drawings and are herein described in detail. It shouldbe understood, however, that the description herein of specificembodiments is not intended to limit the invention to the particularforms disclosed, but on the contrary, the invention is to cover allmodifications, equivalents, and alternatives falling within the spiritand scope of the invention as defined herein.

We claim:
 1. An apparatus for performing somatosensory evoked potentialmonitoring during surgery, comprising: a control unit, said control unitbeing configured to: receive evoked somatosensory response data based ontransmission of a plurality of electrical stimulation signals to aperipheral nerve of a patient; increase a number of allowed evokedsomatosensory responses from the evoked somatosensory response data bymodifying a noise rejection threshold until the evoked somatosensoryresponse data have a rejection rate that is less than a predeterminedrejection rate value; evaluate a plurality of latency valuescorresponding to the allowed evoked somatosensory responses from theevoked somatosensory response data; based on (i) the evaluationsatisfying a stability threshold and (ii) a comparison of an amplitudevalue of the evoked somatosensory response data to an amplitude value ofthe modified noise rejection threshold, refining at least onestimulation signal parameter that corresponds to at least one of theplurality of electrical stimulation signals; and direct transmission ofsubsequent electrical stimulation signals using the at least one refinedstimulation signal parameter.
 2. The apparatus of claim 1, wherein thecontrol unit is further configured to receive instructions from a userto modify the at least one stimulation signal parameter.
 3. Theapparatus of claim 1, wherein the at least one stimulation signalparameter is at least one of stimulation rate, current, pulse width, andpolarity.
 4. The apparatus of claim 3, wherein the control unit isfurther configured to refine the at least one stimulation signalparameter automatically.
 5. The apparatus of claim 4, wherein thecontrol unit is further configured to perform a threshold huntingalgorithm to refine the subsequent electrical stimulation signals. 6.The apparatus of claim 1, wherein the control unit is further configuredto perform at least one of stimulated electromyography and motor evokedpotential assessments.
 7. The apparatus of claim 1, wherein the controlunit is further configured to generate an alert and provide instructionsto display a cause for an alert condition to a user.
 8. The apparatus ofclaim 7, wherein the cause for the alert condition relates to at leastone of anesthetic, cortical ischemia, muscle activity artifact,mechanical insult, and an electrode issue.
 9. The apparatus of claim 7,wherein the cause for the alert condition is at least partiallyattributable to ischemia of at least one of the spinal cord and brain ofthe patient.
 10. The apparatus of claim 7, wherein the cause for thealert condition is at least partially attributable to an electrodeissue, and the optimal noise rejection threshold is at least 100 μV. 11.The apparatus of claim 7, wherein the cause for the alert condition isat least partially attributable to a mechanical insult to the spinalcord of the patient.
 12. The apparatus of claim 7, wherein the cause forthe alert condition is at least partially attributable to a degradationof a somatosensory response along at least one limb of the patient. 13.A method for performing somatosensory evoked potential monitoring duringsurgery, the method comprising: receiving evoked somatosensory responsedata based on transmission of a plurality of electrical stimulationsignals to a peripheral nerve of a patient; increasing a number ofallowed evoked somatosensory responses from the evoked somatosensoryresponse data by modifying a noise rejection threshold until the evokedsomatosensory response data have a rejection rate that is less than apredetermined rejection rate value; evaluating a plurality of latencyvalues corresponding to the allowed evoked somatosensory responses fromthe evoked somatosensory response data; based on (i) the evaluationsatisfying a stability threshold and (ii) a comparison of an amplitudevalue of the evoked somatosensory response data to an amplitude value ofthe modified noise rejection threshold, refining at least onestimulation signal parameter that corresponds to at least one of theplurality of electrical stimulation signals; and directing transmissionof subsequent electrical stimulation signals using the at least onerefined stimulation signal parameter.
 14. The method of claim 13,wherein the at least one stimulation signal parameter is at least one ofstimulation rate, current, pulse width, and polarity.
 15. The method ofclaim 14, wherein refining the at least one stimulation signal parametercomprises refining the at least one stimulation signal parameterautomatically.
 16. The method of claim 15, wherein refining the at leastone stimulation signal parameter comprises performing a thresholdhunting algorithm to refine the subsequent electrical stimulationsignals.
 17. A method for performing somatosensory evoked potentialmonitoring during surgery, the method comprising: receiving evokedsomatosensory response data based on transmission of a plurality ofelectrical stimulation signals to a peripheral nerve of a patient;increasing a number of allowed evoked somatosensory responses from theevoked somatosensory response data by modifying a noise rejectionthreshold until the evoked somatosensory response data have a rejectionrate that is less than 30%; evaluating a plurality of latency valuescorresponding to the allowed evoked somatosensory responses from theevoked somatosensory response data; based on (i) the evaluationsatisfying a stability threshold and (ii) a comparison of an amplitudevalue of the evoked somatosensory response data to an amplitude value ofthe modified noise rejection threshold, refining at least onestimulation signal parameter that corresponds to at least one of theplurality of electrical stimulation signals; and directing transmissionof subsequent electrical stimulation signals using the at least onerefined stimulation signal parameter.
 18. The method of claim 17,wherein the at least one stimulation signal parameter is at least one ofstimulation rate, current, pulse width, and polarity.
 19. The method ofclaim 18, wherein refining the at least one stimulation signal parametercomprises refining the at least one stimulation signal parameterautomatically.
 20. The method of claim 19, wherein refining the at leastone stimulation signal parameter comprises performing a thresholdhunting algorithm to refine the subsequent electrical stimulationsignals.